This invention relates to a window wiper apparatus for a motor vehicle, and particularly to a two speed window wiper apparatus using a three brush permanent magnet motor. In such apparatus, the three brush motor includes two conventional brushes 180 electrical degrees apart, which are called the low speed brush and common brush, and further includes a third brush, called the high speed brush, which is offset by a predetermined electrical angle from the low speed brush so as to operate the motor and wiper at a higher speed and lower torque when the electrical power is applied across the high speed and common brushes than when it is applied across the low speed and common brushes. Such motors have been used in wiper systems for many years.
Wiper systems for vehicles must be operable over a wide range of torques and speeds, since production variability in the wiper drive mechanism and blade friction lead to differing torques and speeds under identical conditions from one vehicle to the next and, in addition, the design of the wiper drive mechanism produces a wide range of torque and speed throughout a single wipe cycle for a single vehicle. Governmental requirements dictate a minimum average speed for a wipe cycle in high speed operation; and, with the variability of torque and speed throughout the cycle, the peak speed during the cycle is significantly greater than the average speed for the cycle. When a margin is provided to ensure the required average speed, it can be necessary, for some designs, to accept the possibility of very high peak speeds of the wiper blades.
Governmental requirements also dictate a minimum wipe area, expressed as a percentage of the total window area. As motor vehicles are being designed for lower air resistance, their body shapes are becoming lower and more streamlined. This leads to larger windshields with a greater required surface area to be wiped. The larger wipe area requires larger wiper blade mechanisms with longer arms and blades; and this produces more inertial mass and greater blade tip speeds when driven with the same wiper motors in the same manner as smaller wiper mechanisms. This effect is made even greater by the fact that governmental average speed requirements are specified in terms of wipes per minute; and actual blade peak speeds increase with increased arm and blade length for a constant number of wipes per minute.
The increased blade peak speeds and inertial masses lead to more difficulty in reversing the blades at each end of the wipe pattern without overshooting and hitting the window frame. They also lead to greater strain on the wiper blade and drive mechanism in wiper operation at high speed. These effects will become of increasing concern as vehicles become lower with larger windshields.